How exactly is seating depth changes causing the tightening of group size?

[necchi]

Personally, I don't care about the why of it.

Well that's too bad, while your single approach seems to have worked for you, having a basic and fundamental understanding of why and how things work could lead you to becoming more proficient in the procedure if your ever faced with using componants beyond your single experience.

Sorry if you've missed the point of the OP, it really is about expanding knowledge and experiance.

 

[bozo699]

Personally, I don't care about the why of it.

Well that's too bad, while your single approach seems to have worked for you, having a basic and fundamental understanding of why and how things work could lead you to becoming more proficient in the procedure if your ever faced with using componants beyond your single experience.

Sorry if you've missed the point of the OP, it really is about expanding knowledge and experiance.

Spot on Necchi,...100% spot on, why would any of us even bother if it was all that easy!! I mean a lee scoop would be close enough for powder charge, why separate case brands or number of firings, for that matter a 100 grain bullet is a 100 grain bullet,.....isn't it? so if you get a remington core lock mixed up with your Bergers so what, they will still shoot okay :
Wayne.

 

[Steve Wilson]

The -.025 shot lights out. Surprised the hell out of me. I didn't measure them, but at 200 yards they only made one hole.

 

[Steve Blair]

This exact thing happened to me on Tuesday when I was playing with seating depth, on a proven accurate load in my 6 BR. I made up 10 rounds with the bullet +.015 into the lands, 10 at .010 off (my proven load) and 10 at -.025 off. First shot with the +.015 length gave me a sticky bolt. DAMN, my press is at home. If it was with me, I would have just seated one of them a little shorter..... +.010 in and see how the bolt felt, kept going shorter until the bolt lift was fine. In short, I would have been able to work with the seating length right then.

[br]
Couldn't you just take a Wilson seater and a rubber mallet or small arbor press to the range? Although I don't load at the range, those items don't seem too burdensome.

 

[mikecr]

Steve, this is where QuickLoad software comes into value. It never should have been a surprise that going into the lands would significantly increase pressure. Coming in and out of land contact is NOT fine tuning. It's a major shift in window.
You'd have seen this(pressure) using QL, before loading for the test, & could have backed off the charge to closely compensate.

I just don't see loading at the range as beneficial, or as precise.
And given that I pretty much know the pressures before loading to them, even when testing for my max, I don't end up pulling many bullets.

 

[Steve Blair]

Mike, I agree. Steve's approach is not the way I do it, but his dilemma seemed to have an easy resolution. My approach is to maintain a database of past firing data with which I can characterize proposed loads and reasonably predict results. If I have a new powder, for instance, I will load several charges within a safe range and fire them to collect velocity data. Knowing that, I can make assumptions about behavior using QuickLOAD that get me pretty close to actual results. Some powders have a roughly linear pressure increase with charge, others show a more upward trajectory. My past results allow me to take all this into account to minimize experimental firing. I do not fire a single handload in any gun without first modeling it in QuickLOAD.

 

[CZ550]

I assume that most of this discussion is based on jumping the bullet. With that said if you seat the bullet out further say 5 thou but still jumping to the lands. Pressure should decrease because of an increase in case capacity if charge remains the same. Therefore barrel time might actually increase because of decrease in pressure. Guess what I am saying if the nodes are velocity or (barrel exit time) related. Leaving the seating depth alone and adjusting the charge should achieve same results.

For example if +5 thou shoots better, wouldn't lowering the charge and leaving the seating depth alone be the same. ?

I have always looked at seating depth as a condition that some bullet designs require to shoot better. Some need to start in a Jam condition some require Clearance from the lands. And after I determine the starting condition of the bullet I am using its all about developing pressure with powder.

That's how I look at it and I am sure I am wrong on several accounts but it makes sense to me.

 

[nonliberal]

I just don't see loading at the range as beneficial, or as precise.
And given that I pretty much know the pressures before loading to them, even when testing for my max, I don't end up pulling many bullets.

In score shooting I started loading at the range for just one reason, it is easier for me to keep tabs on 10 identical cases than 100 cases. I'm always afraid a piece of brass is going to slip through the cracks with a donut or more or less spring back, ect, and often times dropping one point looses the match.

I weigh all my charges and keep them in clear test tubes with caps so its a very quick process to dump perfect charges at the range.
I really don't make any load adjustments at the range.

 

[Shynloco]

I'm lovin this discussion about "theories" about depth seatings and the effects it has on tightening group size. And let me add I don't have the answer with several of you that sound like you have very credible arguments to make. But even if those theories are correct, how come there isn't a constant which means that seating into the lands doesn't work on every rifle? That I fear that opens up a whole new can of worms that leads to harmonic differences, barrel lengths, barrel quality, etc, etc, etc! Man I love this stuff, but I'm still gonna have to test every new setup or barrel put on any of my rifles to see exactly which "theory" applies.

 

[DougMH]

Personally, I don't care about the why of it.

Well that's too bad, while your single approach seems to have worked for you, having a basic and fundamental understanding of why and how things work could lead you to becoming more proficient in the procedure if your ever faced with using componants beyond your single experience.

Sorry if you've missed the point of the OP, it really is about expanding knowledge and experiance.

I think you missed my point. I didn't mean that jump, OTL and jam won't make a difference, I just meant that I don't care why it makes a difference. I just want the smallest vertical/horizontal to come out of my testing with it. What difference does it make if it's because of the way a jump impacts the lands, the way a jam meshes with the lands, barrel harmonics, or any other theory. That's going to be different with every single barrel produced... even those from the same barrel maker. That's just just stuff to "bench talk" about. You just have to test with it until you get the best results.

 

[necchi]

I just meant that I don't care why it makes a difference.

Oh well, that's seems to be a problem when a topic get's into it's 3rd page or beyond, the original post and all the info on page 1-2 is forgotten and it takes on another life of it's own.

It's actually a good topic for those that have followed it.

 

[bozo699]

I just meant that I don't care why it makes a difference.

Oh well, that's seems to be a problem when a topic get's into it's 3rd page or beyond, the original post and all the info on page 1-2 is forgotten and it takes on another life of it's own.

It's actually a good topic for those that have followed it.

Absolutely!!
Wayne.

 

[Bradley Walker]

The -.025 shot lights out. Surprised the hell out of me. I didn't measure them, but at 200 yards they only made one hole.

Uh huh...

 

[Bradley Walker]

Until someone can show what exactly bullet seating does to shot to shot consistency, and why... (and not a "theory"), I am going to stand by my assertion that bullet seating depth is a function of the bullet deign itself.

I'm still not ready to buy into this "bullet timing" thing being seating depth dependent... I am sorry it sounds like hogwash to me.

There was a guy on here that set a new World's record for smallest group for 1000 yards, something like 1.4"??? Ummmm... the barrel was brand new and he was shooing his same load from before?

I would think that REPRODUCING seating depth would be important, and I think the seaters (seater stems) themselves leave a lot to be desired in that respect....

 

[BoydAllen]

Bradley,
I don't believe that there is only one factor. Yes, bullet shape is one. There are others. Are you aware of tuning nodes for powder charge weight? A reliable source tells me that his testing has shown that this is the case with seating depth as well. Fellows who play with tuners will tell you that sweet spots repeat at intervals. IMO what these all have in common is that they either effect the timing of when the bullet exits the muzzle, relative to where the muzzle is in its vibration cycle, or they change the cycle frequency to be a better match with a given load. None of this contradicts the idea that different bullet shapes have different seating depth preferences, or that you can use what you already know about where a bullet, or a similar bullet's preferences as an educated starting point for finding peak tune. I do that all the time...and it works.

 

[queen_stick]

Boyd... I think you'll remember the two guys that practice at my local range (2 very good short-range BR shooters)... one of them told me something very interesting that last time I spoke to him... tuning with bullet seating affects your groups vertically and horizontally... Most people blame the wind (myself included), but according to him, it could be bullet seating causing horizontal dispersion. That's why those guys use a sharpie and write the optimum bullet seating depth right on the side of their barrels. Once they find it, they do NOT change it.

I'm still chewing on that theory...

 

[jlow]

I used to think that, but was corrected by someone recently and I agree with the correction. It makes little sense that the vibration of the barrel would only go up and down and not sideways….

 

[queen_stick]

to clarify... In no way did I disagree with my source (see below)... it's just a lot to take in when you consider everything going on. You better have EVERYTHING working correctly (including yourself) if you plan to tune bullet seating depth to reduce horizontal dispersion on the target.

results like this don't happen on accident... http://bulletin.accurateshooter.com/2008/08/ibs-100200-national-results-harley-baker-wins-big/
When he speaks, I tend to listen
My other "source" is also listed in those results in the link above... multiple times.

I consider myself lucky to run into those guys at the range occasionally.

 

[mikecr]

You can see in any ladder, or seating testing, that there is clearly horizontal and not just vertical.
This still makes 'some' sense w/regard to timing as there are multiple things going on in a barrel at once;
Whip (conventional tuning)
Bore dimension wave (OBT)
Pressure tune (OCW)
Bore torquing/snap back (design factor)
Vibrations

 

[queen_stick]

what's OBT ?